Electrometer amplifier tube



Feb. 1, 1955 J. H. COLEMAN ELECTROMETER AMPLIFIER TUBE 2 Sheets-Sheet 1Filed Nov. 20, 1950 l/ll/I/II INVENTOR.

I/IIIIIl/I BY K (DR-Ma... imwfi Feb. 1, 1955 J. H. COLEMAN 2,701,319

ELECTROMETER AMPLIFIER TUBE Filed Nov. 20, 1950 2 Sheets-Sheet 2INIVENTOR.

United States Patent its! ELECTROMETER ANIPLIFIER TUBE John H. Coleman,Palm Beach, Fla., assignor to Radiation Research Corporation, West PalmBeach, Fla., a corporation of Florida Application November 20, 1950,Serial No. 1%,553

13 Claims. (Cl. 313-69) This invention relates generally to electrometeramplifier tubes, and more particularly to a unique and new combinationof tube elements with a deflection type control providing highsensitivity and low control electrode currents.

In practical operations, ordinary amplifier tubes fail to approach therequirements of an electrometer tube, defined by Victoreen Electrometertubes for measurement of small currents, Proceedings of the Institute ofRadio Engineers, volume 37, page 432, 1949, as a tube in which thecontrol element has a leakage current of less than amperes.

In conventional vacuum cathode tubes, the limitations on the controlgrid resistance have been found, as reported by G. F. Metcalf and B. T.Thompson, A low grid current vacuum tube, Physical Review, volume 36,page 1489, 1930, to be current from the following factors: (1) positiveions formed by collisions of the electron beam with gas in the tube; (2)ions emitted by the filament; (3) photo-electrons emitted by the controlgrid under'action of light from the filament; (4) photo-electronsemitted by the control grid under action of soft X-rays produced by thenormal anode current.

After careful processing, as reported by Victoreen, a standard tetrodecan be made to operate with low control grid current by adjusting theplate voltage to a low value to eliminate ionization (factor 1) and softX-rays (factor 4); and by operating the first grid at a slightlypositive potential to repel positive ions from the cathode (factor 2);and by operating the cathode at a low temperature to reducephoto-electric emission from the control grid (factor 3). The voltagegain for this type of operation is less than unity;however, the currentamplification is sufficient to operate an additional standard amplifier,which is necessary and required, however, to measure and record lowcurrent.

Among the prior approaches to the control of electron streams are thebeam deflection type amplifiers, in which instead of changing thecurrent density of the electron stream as does the grid of an ordinaryvacuum amplifier, the control electrode of the tube electricallydeflects the electron stream between an array of collecting plates; theelectron ray tube, which is similar to the deflection tube in that thecontrol elements change the direction of the electron paths in the beam,but differs from the deflection tube in that a fluorescent screen isused to indicate the change in beam direction instead of a system ofcollector plates; the post deflection-acceleration cathode ray tube,which is similar to the electron ray tube except for the beam which issharply focused to a spot instead of being a Wide angle stream; and,tubes employing positive ion trapping in a field-free region for spacecharge neutralization to given a sharply focused, high current beam. Oneof these tubes was used as an amplifier by varying the amount oftrapping by electrically removing some ions from the field-free regionby the application of the signal as a sweeping out potential to vary thebeam density accordingly.

In all of the deflection tubes, electron ray tubes, and cathode raytubes described in the preceding paragraph, the control electrodecurrent is excessive due to the positive ions formed in the long beampath. The long path increases deflection sensitivity in directproportion to length; however, the path for ionization is also increasedproportionately. The spacecharge neutralized amplifier 0 wasfundamentally unsuited for low current work because .1

2,701,319 Patented Feb. 1, 1955 the control electrode had to collectcurrent for the operation of the tube.

The present invention, on the other hand, by a unique and newcombination of the beam deflecting principle and positive ion trapping,provides a high deflection sensitivity by a long drift space for theelectrons after deflection, and provides a low control electrode currentby retaining in the drift space all positive ions formed therein. Also,a special combination of radiation shielding for the control elementswith positive ion trapping is provided to further sensitivity.

, Accordingly, an object of this invention is to provide an improvedmethod of measuring small currents, and an improved electrometeramplifier tube to measure small currents.

Another object of this invention is to provide means for measuring smallcurrents comprising a source of electrons in an evacuated medium, anelectrical system for buiding up a potential in accordance with thesmall current, varying the direction of the electron trajectories inaccordance with the potential on the said electrical system, projectingthe electron stream through a second and substantially field-free regionof said medium whereby the space charge within the beam is neutralizedby positive ions, and colleeting the electron stream in such a manner asto indicate visibly or electrically the magnitude of the change indirection of the electron trajectories and consequently the magnitude ofthe impressed small current.

Another object of this invention is to provide in combination with themeans aforesaid, means to shield the control electrode from internallygenerated radiation, such as photo-electrons from the cathode and X-raysfrom the bombardment of the collecting anode by the electron stream;and/ or means for electro-statically repelling positive ion emissionfrom the source of electrons away from the deflection control region;and/or means for electrostatically shielding the deflection controlregion from positive ions formed by collision of the electron streamwith neutral gas particles in the control region.

A still further object of this invention is to provide a single tubedetection and measuring system for small currents.

The foregoing and other objects and advantages of this invention will bemore apparent from the following description, in conjunction with thedrawings, forming a part thereof, wherein:

Fig. 1 is a transverse cross-sectional view of the tube showing thebasic elements of this invention with collector plates as an outputsystem;

Fig. 2 is a transverse cross-sectional view of the tube showing thebasic elements of this invention with three control electrodes and afluorescent screenas a visual output system;

Fig. 3 is a cross-sectional view of a longitudinal section of the tubeshowing the basic elements of the invention with end plates providingaxial deflection, electrical lead lines shown in elevation;

Fig. 4 is a view similar to Fig. 3 with a fluorescent screen as a visualoutput system;

Fig. 5 is a transverse cross-sectional view of the basic elements of theinvention employing an outer electrode with a plurality of controlelectrodes and collector plates;

Fig. 6 is a cross-sectional view of a longitudinal section of the tubeshowing a frustro-conical shaped fluorescent screen as a visual outputsystem, electrical lead lines shown in elevation;

Fig. 7 is a cross-sectional view similar to Fig. 1 with a plurality ofcontrol electrodes and two collector plate systems;

Fig. 8 is a cross-sectional view of a longitudinal section of the tubeshowing a modification of the invention emloying an outer cathode withaxial deflecting plates and a two system collecting anode, electricallead lines shown in elevation;

Fig. 9 is a cross-sectional View of some elements of the inventionshowing the location of a space charge grid to repel positive ions fromthe cathode;

Fig. 10 is a view similar to Fig. 5 modified to show shields betweencollector plates and control electrodes and between control electrodesand the cathode;

Fig. 11 is a cross-sectional view of part of the elements of thisinvention showing the use of a screen grid to provide electro-staticshielding of the control elements from the accelerating potentials onthe trapping grid;

Fig. 12 is a cross-sectional view taken along the transverse line 12+12of Fig. 11.

Referring to the drawings, wherein like members are given the samereference numeral, a cathode .1, usually and preferably cylindrical inform, has spaced therearound or equi-distant therefrom a plurality ofcontrol electrodes 2, specifically 2 2 22 2 2 2 2 The cathode 1 providesa source of electrons which are attracted by a positive potential on theaccelerating and trapping grid mesh 3 and .on to the collecting member5, specifically '5 "55 5 5 5 5 and 10, which may be collector plates inone embodiment of the invention and a fluorescent screenin-anotherembodimentof the invention. The grid mesh 3 is usually in the form of acylinder and positioned intermediatethe control electrodes 2 and thecollecting members 5 with which the grid mesh 3 forms a field-freeregion 18 for positive ion trapping. In the case of the fluorescentscreen 7 the collector 5 is electrically connected to the trapping grid3. In the case in which collecting plates 5 are used, the individualplates can be operated at the same potential as the trapping grid 3;however, separate leads are brought out to indicate the collectedcurrent. The elements are enclosed in a glass or other transparentvacuum envelope 6, having the usual and necessary electrical lead linessealed therethrough and therein. These are the basic elements of theinvention, and the-additions, modifications, and changes will bedescribed with respect to the particular drawing illustrating them. Itis understood, however, that the invention is not limited to theadditions and modifications described, as they are shown in illustrationand not in limitation.

In Fig. 1, the cathode 1 is cylindrical in form and positioned along thelongitudinal axis of the tube 6 and emits electrons, which areaccelerated by the positive potential on the grid mesh 3 of any suitableconductor material permeable to electrons and attracted to thecollecting plates 5 and 5', being semi-cylindrical. The grid mesh 3 is acylinder, preferably of tungsten or molybdenum, and connected on eachend by electrical conductors to enclose the region between the grid mesh3 and the collecting plates 5 and 5 The distribution of the current ofthe electron stream 17, defined in the drawing between the lines thereshown, to the collecting plates 5 and 5 is determined by the relativedeflecting potentials on the control electrodes 2 and 2'. An increase innegative potential on 2 for example, increases the current to theopposite collecting plate 5 be attached to the collecting plates 5 and 5with any type of load in conventional beam deflection plate circuits.When the accelerating, trapping grid 3 and collecting plates 5 areoperated at the same potential, the potential of the inter-electrodespace '18 remains negative with respect to these electrode potentialsdue to the etfect of the negative charges of the electron stream 17. Themutual repulsion of the negative charges ordinarily re- .sults in alimitation in the current density of the electron stream 17; however,positive ions formed by collision of the electrons with the neutral gasare attracted into the electron stream 17 until the space chargedepression of potential is neutralized, resulting in two favorableeffects. First, the positive ions formed in the inter-electrode space ortrapping space 18 do not pass through the trapping grid 3 to the controlelectrodes 2; and, secondly, deflection sensitvity of the .controlelectrodes 2. is increased by the removal of current density limitationsin the trapping or drift space 18 of the beam or electron stream '17before impinging upon the collecting plates 5.

In Fig. 2, two modifications of Fig. l are shown. First a fluorescentscreen 7 is employed to indicate visually the variation in direction ofthe electron stream 17 in accordance with the potential on the controlelectrodes 2 by fluorescence. The collector S can be evaporated on aphosphor, according to the teaching of D. W. Epstein and L. Pensak, RCAReview, volume 7, page 5, March 1946, which phosphor can then bedeposited directly upon the envelope 6. Secondly, a third controlelectrode has been added. The spacing between the control electrodes 2 2and 2 can be variedto give varying degrees of sensitivity. For example,in the drawing the electron stream 17 pass ing between the closestspaced electrodes, 2 and 2 is the most sensitive in deflection to thecontrol voltage.

There are several additions and modifications of the Conducting leadscan invention as illustrated in Fig. 3. The electron stream 17 emittedby the cylindrical cathode 1 is deflected axially between thecylindrical collector plates 5 and 5 by the potential on the controlelectrodes, the disc shaped one 2 and annular ring 2 An increase innegative potential on the control electrode 2 results in an increase inthe current to 5 The positive ion trapping is enhanced by providingtrapping grid plates 12, in the form of annular rings attached to therespective ends of the grid mesh 3 and extending at least to the wall ofthe cylindrical collector plates 5 to which they are electricallyconnected, thus completing the electrical enclosure of the field-freespace. Some details of the tube construction are shown in the drawing,such as the filament leads 8 of the cathode 1 being brought down to thetube base. Also, the leads to the control elements 2 can be brought outthe top of the envelope 6 to provide a long surface leakage path overthe glass envelope 6.

In Fig. 4 the axial deflection system illustrated in Fig. 3 is combinedwith the visual indication illustrated in Fig. 2. In this modification,the collector and phosphor combination isdeposited upon a glasscylinder, here designated the collector 5; however the collectorphosphor screen is provided as previously described.

An inverted arrangement of the elements is illustrated in Fig. 5, toprovide a large area cathode 1; this outer cathode 1 is illustrated as acylinder, however, it could be a series of cylindrical cathodes ,or anyother convenient form. The cathode 1 may conveniently have the activecoating only on the surface between the control rods 2. In theembodiment illustrated, the control rods 2 and collector plates 5 areincreased in number to four each; any number of these elements in anydesired shape can be used the same as in the case of the tube with theinner cathode.

A change in visual output screen in the basic embodiment is illustratedin Fig. 6, in which the geometrical design of the phosphor collectorscreen is frusto-conical. In this design of the collector 5, it does nothave to be transparent when the trapping plate 12 is constructed in theform of a grid mesh to permit observation of fluorescence on the side ofthe collector system 5.

In Fig. 7, there is illustrated a modification in the collecting platesystem. The cylindrical cathode 1 is surrounded by a plurality ofcontrol electrodes 2, which are again shielded by the trapping grid meshcylinder 3 from the positive ions formed by the electron stream 17before collection by the plates 5 and 10. The series of collectinganodes 5, equal in number to the control electrodes, are placedequidistant from each other and on an equal radius. The collectingcylinder 10 is on a greater radius than the collecting anodes 5, andemployed therewith to obtain desired field strength at the edges of thecollecting anodes 5. Thus, the electron stream 17 can be deflected bythe control electrodes 2 between the collecting plates 5 and cylinder 10with higher sensitivity than the system illustrated in the previousfigures, when the relative potentials of adjacent electrodes in thecollecting system vary in magnitude with the corresponding current andload of the particular electrode. The collecting anodes decrease inpotential due to the drop across the load in the output circuit;however, one electrode can have a reduction in potential of at least 10%below other potentials before space charge neutralization is lost bysweeping out an appreciable number of positive ions even at lowpressures. If, one the other hand, either collector system were reducedin potential by a sufficient amount to sweep out positive ions, only thedeflection sensitivity would be reduced by space charge limitations. Thecontrol electrode current would not be increased in any case due to thepositive ion potential barrier formed by the trapping grid mesh 3 withrespect to any reduction in the potential of the collecting anodesystems.

The collecting anode arrangement employed as illustrated in Fig. 7 maybe inverted with respect to the other elements as illustrated in Fig. 8.The outer cathode 1, in the form of a cylinder and provided with heatingcoils 9, emits electrons controlled by axial deflecting plates 2,accelerating and trapping grid 3 and trapping grid plates 12electrically connecting the grid 3 with the collecting anodes 5 andcollecting plate 10.

In Fig. 9 there is illustrated the use of the conventional space chargegrid 11 which can be operated at a positive potential to repel positiveions back into the cathode 1. This figure also illustrates a linear typeof beam structure. The cathode 1 andother elements are shown asrectangular; however, any other geometrical design, such as sphericaland cylindrical, can be used as a single unit instead of the multipleunit shown in previous figures.

The basic arrangement of elements as illustrated in Fig. is modifiedinFig. to show additional elements shielding the control electrodes 2and improving sensitivity. The space charge grid 11, in the form of amesh cylinder similar to and of similar material as the trapping grid'3, is positioned intermediate the control electrodes 2 and cathode 1 andprovided with solid strips 15, of lead, tungsten or molybdenum foilopaque to photons, to shield the control electrodes 2 from the photonsemitted from the filament. Although the structure illustrated is similarto that of Fig. 5, any other type of structure may be used and any formof light shields located between the cathode 1 and control electrodes 2as long as the path of the electron stream 17 is not restricted betweenthe cathode 1 and collecting anodes 5. Another modification andimprovement on the basic elements of this invention is also shown inthis drawing, namely the shields 13, similar to the shields 15,positioned on the trapping grid mesh 3 to absorb X-ray radiation fromthe collecting anodes 5 under bombardment by the electron stream 17.These shields can also be located on other electrodes and can have anydesired shape as long as the electron stream 17 is not restricted. Theseshields 13 and 15 may each or both be used with the basic elements ofthe invention.

In Fig. 11 another feature that may be added to the basic elements ofthe invention to determine the output characteristics is illustrated. Ascreen grid mesh 16, similar to the grid meshes 3 and 11, is positionedintermediate the trapping grid mesh 3 and and the control electrodes 2and can be operated at such a potential as to increase deflectionsensitivity, to restrict ionization by the electron stream 17 in thecontrol electrode region, and to focus the positive ions that are formedin the high velocity region between the trapping grid 3 and screen grid16 away from the control electrodes 2. Without the screen grid 16 somepositive ions are formed in the region between the cathode 1 and thetrapping grid 3 or between the trapping grid 3 and the space charge grid11, by the collision of the electron stream 17 with neutral gas in thisregion, despite the close spacing of these electrodes. Normally, some ofthese positive ions will be collected by the negative control electrodes2 despite other forces tending to carry them into the cathode 1. Thus,the screen grid 16 can be operated below the ionization potential of thegaseous medium to restrict any ionization in the control region betweenthe cathode 1 and the screen grid 16. A further advantage of the screengrid 16 is that its geometry can be adjusted to cause the positive ionsformed between the trapping grid 3 and screen grid 16 to be injectedinto the control region between the cathode 1 and the screen grid 16with the control electrodes 2 a less favorable target. The cylindricalstructure, better illustrated in Fig. 12, is an example of a geometricaldesign which tends to focus the positive ions away from the controlelectrodes 2 into the cathode 1 where they are collected as current.

This invention eliminates the necessity for using two tubes inelectrometer work to obtain any voltage amplification of small currentsbecause the plate voltage is not restricted.

The tube as illustrated and described herein can be used with anystandard amplifier circuit. When it is used in the measurement of smallcurrents, this tube eliminates the necessity of an additional amplifiertube to obtain voltage gain, but is electrically connected in thecircuit the same as an amplifier tube would be connected.

The mutual conductance and amplification factor of the tube of thisinvention is determined by the relative dimensions and geometry of thetube elements, and can be determined by conventional calculations.Extending the drift or field-free space increases the mutualconductance.

A practical application of the invention is in the detection of smallcurrents encountered in nuclear work. For example, a visual type tubecould be used with an ionization chamber as a simple radioactivedetector; and meters can be used in the output circuit for operationdirectly as a sensitive electrometer.

Having thus described the invention, what is claimed as new and desiredto secureby grant of United States Letters Patent is:

1. In a vacuum tube the combination comprising a cathode, controlelectrodes, a series of collector plates concentrically spacedfrom thecathode and exposed directly to the latter in a field free region, atrapping grid mesh intermediate said collector plates and controlelectrodes and having the same potential as said collector plates, andshields between said cathode and collector plates to absorb X-rays ofbombardment in the cathode stream.

2. In a vacuum tube the combination comprising a cathode, controlelectrodes, a series of collector plates concentrically spaced from thecathode and exposed directly to the latter in a field free region, atrapping grid mesh intermediate said collector plates and controlelectrodes and having the same potential as said collector plates, apositive ion repelling grid positioned intermediate said cathode andcontrol electrodes, and X-ray absorbing slliields positionedintermediate said cathode and collector p ates.

3. In a vacuum tube the combination comprising a cathode, controlelectrodes, a series of collector plates concentrically spaced from thecathode and exposed directly to the latter in a field free region, atrapping grid mesh intermediate said collector plates and said controlelectrodes and having at least the potential of said collector plates,photon absorbing shields positioned intermediate said cathode and saidcontrol electrodes, and X-ray absorbing shields positioned intermediatesaid cathode and collector plates.

4. In a vacuum tube the combination comprising a cathode, controlelectrodes, a series of collector plates concentrically spaced from thecathode and exposed directly to the latter in a field free region, apositive ion repelling grid mesh intermediate said control electrodesand said collector plates, X-ray absorbing shields intermediate saidcathode and said collector plates, and a screen grid mesh intermediatesaid control electrodes and positive ion repelling grid mesh.

5. In a vacuum tube the combination comprising a cathode, controlelectrodes, a single collector system, uniformly and concentricallyspaced from the cathode and exposed directly to the latter in a fieldfree region, a positive ion repelling grid intermediate said collectorand said control electrodes, photon absorbing shields positionedintermediate said cathode and control electrodes, X-ray absorbingshields positioned intermediate said cathode and collector system, and ascreen grid mesh intermediate said positive ion repelling grid.

6. In a vacuum tube the combination comprising a cathode, deflectingelectrodes, a series of collecting members, a trapping grid intermediatesaid collecting members and deflecting electrodes, trapping platesintermediate said trapping grid and collecting members and electricallyconnected to said trapping grid, and said trapping grid and collectingmembers having the same potential.

7. An electronic tube comprising an envelope enclosing a cathode, asegmental anode spaced therefrom, deflecting electrodes mountedintermediate said cathode and anode, and a positive ion trappingelectrode intermediate said deflecting electrodes and said anode.

8. An electronic tube as claimed in claim 7 and including a positive ionrepelling grid positioned intermediate said cathode and said deflectingelectrodes to repel positive ions emitted by said cathode.

9. An electronic tube as claimed in claim 7 and including photonabsorbing shields between said cathode and said deflecting electrodes toabsorb photons from the cathode.

10. An electronic tube as claimed in claim 7 and including a screen gridintermediate said positive ion trapping electrode and said deflectingelectrodes to change the electrical field at the deflecting electrodesto prevent positive ions between said cathode and said positive iontrapping electrode returning to said deflecting electrodes.

11. An electronic tube as claimed in claim 7 and including a positiveion repelling grid positioned intermediate said cathode and saiddeflecting electrodes, and photon absorbing shields positionedintermediate said cathode and said deflecting electrodes.

12. An electronic tube as claimed in claim 7, and including a positiveion repelling grid positioned intermediate said cathode and saiddeflecting electrodes, and

a estee gici in ermesl ate sa d p s t ve i n trapping e Re er n 3W i 1.1? fi e 9 t is -,p en trode and e ecting electro es. 7

13- ;An electronic tube as plaimed in ,elaim 7 and in- UNITED STATESPATENTS plugling photon absql bing shields positioned intermediate2,175,700 Roberts Oct. 10, 1939 saidpathode and -s aid defieetipgeleetrodes, and a spreen 5 2,197,041 Gray Apr. 16, 1940 gridintermediate said positive ion itrapping electrode and 2,243,408Anderson et al. May 27, 1941 said deflecting ele etr odes. 2,273,800Schliemann Jensen Feb. 17, 1-942 7 2,278,630 Winter Apr. 7', 19.42

2,311,672 Le Van Feb. 23, 1 943 0 12,470,732 Visscher May 17 1949

